Lens Camera

Part of Photography

A lens camera replaces the pinhole with a glass lens, gathering far more light and enabling exposures measured in seconds rather than minutes — the practical standard for working photography.

Why This Matters

The pinhole camera is the conceptual foundation of photography, but it is not practical for most work. A 0.4 mm pinhole admits perhaps one-millionth the light that a 25 mm lens opening does, making exposures hundreds or thousands of times longer. In bright sun, a pinhole requires 15-30 minutes to properly expose a sensitive plate; a lens camera requires 2-10 seconds. For any moving subject — people, animals, flowing water, wind-blown vegetation — the pinhole is simply useless.

The lens camera makes photography a practical tool for daily documentation. A portrait can be made in seconds. A survey team can photograph twenty sites before noon. A medical photograph can be taken of a patient who cannot hold still for more than a few seconds. The lens camera enables photography at human scale and human pace.

Building a lens camera requires obtaining a lens — the hardest component — but lenses are available from spectacles, telescopes, microscopes, and other optical instruments. Once you have a lens, the camera body is woodworking: a box with a sliding drawer for focusing, a plate holder, and a simple shutter.

Lens Selection and Properties

What makes a lens suitable for photography:

1. It focuses parallel rays (from a distant subject) to a single sharp point — it is converging or convex
2. The image it projects is sharp across the plate area, not just at the center
3. It is large enough to pass meaningful quantities of light

Types of available lenses:

Spectacle lenses: Very common and easy to obtain. Convex (positive diopter) lenses for farsightedness work well. A +2 diopter lens has a focal length of 500 mm; a +4 diopter lens, 250 mm; a +10 diopter lens, 100 mm. Stronger prescriptions = shorter focal length.

Telescope objective lens: The large front element of a telescope is a long-focal-length, high-quality lens. Focal lengths of 300-1000 mm. Excellent optical quality. Requires a long camera body.

Microscope condensers and objectives: Short focal length, high quality. The condenser (the large lower lens) has a focal length of 20-50 mm — requires a very short camera box. Very wide field of view.

Burning glass: A large, simple magnifying glass sold for fire-starting. Typically 50-100 mm diameter, 100-200 mm focal length. Adequate quality for moderate enlargements; some aberration at the edges. Inexpensive and often available.

Finding the focal length: Hold the lens facing a bright window or bright outdoor scene. Hold a piece of white paper behind it (on the side away from the scene). Move the paper until the scene projects sharply onto the paper. The distance from the lens to the paper at best focus is the focal length. Measure this with a ruler.

Lens quality tests:

1. Hold the lens face-up and look at a ceiling lamp through it. If you see sharp, clear reflections and no visible scratches or bubbles in the glass body, quality is adequate.
2. Mount in a camera and photograph a test scene with fine detail (a printed page, a brick wall). Examine the result for edge blur, chromatic fringing (color edges visible in the negative as alternating dark lines at high-contrast edges).
3. A simple single-element lens (burning glass, single spectacle) will show aberrations — the image quality degrades from center to edge. A compound lens (two or more elements cemented or spaced) shows far less aberration.

Camera Body Construction

Design decisions flow from the lens:

1. Box depth = focal length of the lens (for distant subjects, the plate must be placed at the focal distance from the lens)
2. Box width and height = slightly larger than your plate size
3. Focusing adjustment = a sliding inner box (drawer) that moves ±20% of focal length for subjects from 1 meter to infinity

Dimensions example (for a 150 mm focal length lens, 9 × 12 cm plates):

• Outer box exterior: 14 cm wide × 16 cm tall × 18 cm deep
• Inner sliding box (drawer): 11 cm wide × 13 cm tall × 10 cm deep, sliding inside outer box
• When inner box fully retracted: lens-to-plate = 15 cm (infinity focus)
• When inner box extended to maximum: lens-to-plate = 18 cm (focus at ~50 cm)

Lens mount:

1. Drill or chisel a round hole in the front of the outer box, centered, equal to the lens diameter
2. Turn a wooden sleeve (a short tube) whose inner diameter accepts the lens snugly
3. The sleeve fits into the hole and can be glued or nailed in place
4. The lens sits at the front of the sleeve, held by a retaining ring or drop of wax at its edge

Plate holder: At the rear of the sliding inner box, build a groove or channel that accepts a plate holder:

1. Two grooves, 3 mm wide and 4 mm deep, running across the inside rear face of the box
2. The plate holder is a thin frame of wood or tin, slightly smaller than the plate, with the plate held against it by a removable wedge or spring clip
3. A dark slide — a flat wooden panel — sits in front of the plate holder until the moment of exposure

Shutter: For exposures longer than 0.5 seconds: a simple lens cap (leather disc with a tab for removal and replacement). For shorter exposures: a drop shutter (a flap hinged at the top of the lens board that swings past the lens when released).

Focusing

Unlike a pinhole camera (where everything is equally sharp, or equally slightly blurry), a lens camera must be focused for each subject distance. The closer the subject, the further the plate must be from the lens.

Ground glass focusing:

1. Replace the plate holder with a piece of frosted glass (or oiled paper in a frame) at the same distance as the plate
2. Open the aperture to maximum for a bright, sharp focusing image
3. Look at the focusing glass from behind while pointing the camera at the subject
4. Slide the inner box in or out until the image is sharp on the ground glass
5. Lock the box in position with a wedge or thumbscrew
6. Replace the ground glass with the plate holder containing a coated plate

Distance-to-focus table (150 mm focal length lens):

Subject distance	Box extension beyond infinity position
Infinity (landscape, building)	0 mm
5 meters	5 mm
2 meters	15 mm
1 meter	30 mm
0.5 meters	90 mm

Calculate exact positions using: extension = F × F / (D - F), where F is focal length in mm and D is subject distance in mm.

Aperture and Depth of Focus

A lens camera requires aperture control more than a pinhole camera because the large aperture creates a shallow zone of sharp focus. A pinhole is always at its minimum aperture; a lens can be wide open (maximum light, shallow focus) or stopped down (less light, deep focus).

Making aperture stops: See the aperture article for full details. For a lens camera, the stops mount behind the lens in a slot in the lens board.

Depth of focus at different apertures (150 mm lens, focused at 3 m):

Aperture (f-stop)	Near focus limit	Far focus limit
f/4	2.5 m	3.9 m
f/8	2.1 m	5.3 m
f/16	1.7 m	Infinity

For group portraits or street scenes where subjects are at different distances, use f/11 or f/16 to ensure everyone is in focus. For single-subject portraits, f/5.6 or f/8 gives a slightly blurred background that isolates the subject.

Typical Lens Camera Results

With a good lens (spectacle quality or better), a well-coated gelatin bromide plate, and correct exposure:

• Subjects at 3-10 meters: sharp enough to read text on a sign, identify individuals, see fabric texture
• Subjects at 1-3 meters: very sharp; skin pores visible, machine tolerances recordable
• Subjects at 10-50 meters: sharp enough for surveying, architectural documentation, group identification
• Subjects over 100 meters: adequate for landscape orientation and large features; not for fine detail

The lens camera is the practical photographic instrument for civilization documentation. A survey team with three cameras, 50 coated plates per week, and a central darkroom for developing can create a permanent visual record of a region’s geography, people, buildings, crops, and equipment that serves as the foundation for every planning and rebuilding effort.